The electrodes for transparent electronic devices, such as touch screens are typically coated out of transparent conductive oxides (TCO), such as indium tin oxide (ITO). However, ITO coatings suffer from the following shortcomings: (a) being an inorganic material, ITO is brittle and therefore prone to cracking, especially on a flexible substrate; (b) ITO coatings involve vacuum sputtering of ITO from a target to the substrate in a batch process, which is slow and expensive; (c) the refractive index mismatch between ITO and the substrate, whether glass or polymer-based, can lead to unacceptable optical characteristics that often require deposition of additional refractive index matching layer(s) on the substrate at additional expense; and (d) there is some uncertainty around the global supply of indium which can add to the cost of raw material, in opposition to the market's demand for low-cost devices.
The use of touch screens in display devices has an advantage of saving space by integrating a screen and a coordinate input unit, as compared to a key input type according to prior technology. Therefore, recently developed display apparatuses with a touch screen have gained widespread use and there is more demand than ever for effective transparent conductors for touch screen applications.
The majority of today's commercialized touch screens can be broadly classified into two types:
(1) The first type is a resistive touch screen where upper/lower electrode layers are spaced from each other by a spacer and are disposed to contact each other by pressing. When the upper substrate on which the upper electrode layer is formed is pressed by input units such as fingers, pen, and so on, the upper/lower electrode layers produce conduction and the contact coordinates are recognized due to the change in voltage according to the change in resistance value in the controller. Typically the electrodes for resistive touch screens need not be patterned.
(2) The second type is the capacitive touch screen, where the electrodes are typically patterned. The upper substrate on which the first electrode pattern is formed and the lower substrate on which the second electrode pattern is formed are separated from each other by an insulator. When the capacitive touch screen is contacted, for example, by human finger, the electrostatic field of the screen is distorted which can be measured as a change in capacitance. With the popularity of multi-touch portable display devices, which generally use capacitive touch screens, there has been a significant need for further development in capacitive touch screen technology and choice of new materials.
Conductive polymers are suitable alternatives to ITO, which can overcome many of the aforesaid disadvantages of transparent conductive oxides. Conductive polymers do not require expensive vacuum deposition equipment and provide coatings having high flexibility. However, there are still some challenges in the patterning of conducting polymer layers that need to be overcome for use in, e.g., high quality capacitive touch screens. Several methods for patterning conductive polymers have been proposed.
WO97/18944 discloses forming a mask over a conductive polymer and then exposing the masked polymer to a solution that: i) removes the conductive polymer; ii) decreases the conductivity of the conductive polymer; or iii) increases the conductivity of the conductive polymer. The examples use time consuming photoresist patterning methods to form the mask.
Jabbour and Yoshioka discloses ink jetting oxidants to pattern PEDOT (Adv. Mater. 2006, 18, 1307-1312). With ink jet deposition, however, it is difficult to control the degree of oxidation. Incomplete oxidation can lead to ineffective electrical isolation and too much oxidation can create an objectionable discoloration. Further, ink jet deposition can result in poor patterning due to ink splashing and running. Ink jet patterning is also disclosed in EP1054414.
US20040265623 and US20100197085 disclose the use of laser ablation to pattern conductive polymers. Although popular, laser ablation is a relatively slow process that can result in unwanted line visibility, damage in the substrate and conductive residue that can cause shorts.
Hence, there is a need to develop methods for patterning conductive polymers that will create optically acceptable patterns, by robust, manufacturable ways that can be incorporated in high quality displays.